Malphite was the highest winrate in Plat+ for a top laner, granted part of this was also how he was good against Garen/Darius/Fiora/Riven/Gangplank or at least lane against them. He is also one of the best champions to pick against Vayne if you want to straight up reduce her damage and make her cry, Malphite's E lowers attack speed. But then against, Malphite overall counters most AD champions.
Now that stuff is shifting he isn't on top anymore but he still is good. He is a solid laner as is which is why he didn't get mana buffs, the reason his winrate is dropping is those 5 listed above being nerfed or other people being picked to deal with Malphite when he shows up. Vladmir, Shen, Chogath all say hi.
He still is strong right now, but there is a reason he didn't get mana buffs.
He didn't rely on mana pots and he already was a strong champion as is.